1. Field of the Invention
This invention relates to apparatus and methods for repairing a well. More specifically, this invention relates to apparatus and methods for locating, perforating into and plugging a flow channel outside the casing in a well.
2. Description of the Prior Art
In completing a well, a casing string is typically introduced into the wellbore and cemented into place. In addition to providing physical support of the wellbore, a major purpose of the casing is to prevent communication of fluids between subterranean formations. Often, however, fluid communication between formations results after cementing operations are completed because of the presence of longitudinal channels in or next to the cement sheath.
During a cementing operation, cement channels are frequently formed when the cement slurry fails to uniformly displace the drilling and from all parts of the annulus between the casing and the wellbore. These channels in the cement sheath or in the remaining gelled mud, provide paths for fluid communication between the desired hydrocarbon producing zone and a zone containing water or gas. Such fluid communication may cause several problems, including a reduced producing rate as well as water and gas separation problems afterwards.
To prevent interzone fluid flow, an attempt is usually made to repair the well by a technique known as "squeeze cementing". Squeeze cementing involves randomly perforating the casing at depth in the well where the channel is believed to exist, and injecting cement under pressure into the resulting perforations with the hope that the cement enters and plugs the channel.
A problem associated with squeeze cementing techniques has been that of precisely locating the flow channel. A variety of well logging techniques, including temperature logging, sound logging and radioactive logging methods, have been used in determining the vertical location of a flow channel, but have not been used to determine the precise circumferential location about the casing.
It is presently believed that many channels behind casing exist as relatively narrow channels, such that random perforation according to prior art techniques may not penetrate the channel. Thus, most of the prior methods for plugging channels behind casing often fail to stop fluid communication between zones because the precise location, i.e. a circumferential direction, of the channel is not known. Merely locating a channel at a given depth does not ensure that the channel will be penetrated upon perforation of the casing.